The ability of human beings to maintain stability and balance is controlled by the vestibular system. This system provides the central nervous system with the information needed to maintain balance and stability.
FIG. 1 is a diagram showing the vestibular system. As shown, the vestibular system includes a set of ring-shaped tubes, referred to as the semicircular canals 102a-c, that are filled with the endolymph fluid. The semicircular canals are formed by a membrane called the membranous labyrinth. Each of the semicircular canals 102a-c is disposed inside a hollow bony tube (not shown in the diagram) called the bony labyrinth that extends along the contours of the semicircular canals. Lining the interior walls of the bony labyrinth is a thin membrane called the endosteum. The bony labyrinth is filled with a fluid called the perilymph. As further shown in FIG. 1, each semicircular canal 102a-c terminates in an enlarged balloon-shaped section called the ampulla (marked 104a-c in FIG. 1). Inside each ampulla is the cupula 106a-c, on which hair cells are embedded. Generally, as the semicircular canals 102a-c rotate due to rotational motion of a head, the endolymph fluid inside the canal will lag behind the moving canals, and thus cause the hair cells on the cupula to bend and deform. The deformed hair cells stimulate nerves attached to the hair cells, resulting in the generation of nerve signals that are sent to the central nervous system. These signals are decoded to provide the central nervous system with motion information. The three canals are mutually orthogonal and together provide information about rotation in all three spatial dimensions. The other endorgans in the vestibular system are the otolith organs, the utricle and the saccule. These endorgans act as linear accelerometers and respond to both linear motion and gravity.
In response to the vestibular nerve impulses, the central nervous system experiences motion perception and controls the movement of various muscles thereby enabling the body to maintain its balance.
When some hair cells of peripheral vestibular system are damaged, but others remain viable (as often happens in situations involving bilateral vestibular hypofunction), a person's ability to maintain stability and balance will be compromised. Persons with improperly functioning vestibular systems may consequently experience vertigo, dizziness, and clumsiness, which may lead to collisions and spontaneous falls.
To remedy damaged peripheral vestibular systems, prostheses based on electrical stimulation are being developed. Such prostheses use implanted or non-implanted transmitting electrodes to cause electric stimulation of a target nerve (e.g., vestibular nerve ganglion cells). Such electric stimulation results, for example, in corresponding reflexive responses in the vestibulo-ocular and the vestibulo-spinal pathways, thereby enabling the person to maintain balance and stability in response to the electrical stimulation. Alternatively, such electrodes can target nerves not located in the vestibular system. Electrical stimulation does not involve the actuation of the peripheral vestibular system's hair cells, and thus this type of stimulation lacks a natural feel and makes a person's adaptation to this type of stimulation more difficult.